Let's say you were shooting a visual approach and you were precisely on a three mile final. You gave a glance at the PAPI, you find it giving three white and one red. How high you are from the 3 degree glide path at the moment?

Anybody can do math for me?

  • $\begingroup$ Depends on the angle to which the PAPI is aligned. $\endgroup$ Commented Jan 31, 2019 at 16:12

1 Answer 1


According to AIM 2-1-4, three white one red would equal 3.2 degrees above the horizontal. Doing some math, and assuming you are 3 NM from the TDZ (ie the PAPI) and ignoring slant range error, 3 NM would put you 18,228 ft from the threshold. Assuming you were on the 3 degree PAPI glideslope, you would then be 955.3 ft above TDZE. If .2 degrees high, or 3 white one red, you would be at 1019.1 ft above TDZE, or about 64 ft above the correct glideslope.

Math if you care: [tan(3.2)*18,228]-[tan(3)*18,228].

  • $\begingroup$ Bravo! Thanks :) $\endgroup$ Commented Jan 31, 2019 at 7:53
  • $\begingroup$ That depends on the angle of the PAPI though. Not all runways have the same approach angle. $\endgroup$ Commented Jan 31, 2019 at 16:12
  • $\begingroup$ @J.Hougaard while this is the case, I was working off the information available in AIM 2-1-4, which states the PAPI glidepath is 3.00 degrees. $\endgroup$
    – PilotDan
    Commented Jan 31, 2019 at 18:06

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